In the past missions, you don't obtain Pokémon for your browser through just capturing them. Instead, you need to capture them and they'll have a chance of dropping a Litograph. These litographs then have their own chances of being one of a certain number of Pokémon. Below is a list of all the Pokémon that drop Litographs, their locations as well as the Pokémon you can obtain through the litographs and the rarity of that Pokémon.